|Lange, H; Lischeid, G; Hoch, R; Hauhs, M: Water flow paths and residence times in a small headwater catchment at Gardsjön (Sweden) during steady state stormflow conditions, Water Resources Research, 32(6), 1689-1698 (1996), doi:10.1029/96WR00501|
Since April 1991 the small forested headwater catchment G1 at Gardsjön (Sweden) has been covered by a roof underneath which natural throughfall is replaced by artificial irrigation with a controlled chemical composition. Here this unique experimental setup was used for a tracer experiment with LiBr. The tracer pulse was applied to a subcatchment of approximately l000 m2 that was maintained at steady stateflow conditions throughout the experiment. Except these steady state flow conditions, the irrigation rates corresponded to a typical storm flow episode. Infiltration of event water was confined to the steep slope of a subcatchment of G1; no water was applied at the boggy valley bottom or close to the weir. An array of groundwater wells, suction lysimeters, and surface water sampling plots was used to document the soil passage of this pulse. Breakthrough in runoff (i.e., streamflow from the weir) occurred in a single peak within about 17 hours when less than 15% of the estimated total soil water in the subcatchment was replaced, indicating a relatively small fraction of mobile water. Tracer concentrations in groundwater wells and surface water in the catchment revealed some shallow, locally confined flow paths through the lower parts of the subcatchment. However, 4 days after application of the tracer the runoff concentration already reached the preevent background level for Br-. Only about 14% of the applied tracer was recovered by this time. Taking this into account, i.e., considering bromide as a nonconservative tracer, a one-dimensional model application (two-region convection dispersion approach) successfully reproduced breakthrough curves at various places in the catchment. Thus the small portion of mobile water has intensive contact with the resident immobile water along the one-dimensional flow paths yielding an extremely long tail in the residence time distribution by back diffusion from immobile water. Results of this experiment qualitatively confirm earlier tracer studies under less controlled conditions but are virtually impossible to extrapolate quantitatively to transient flow conditions.